Existing in-vitro nuclear medicine procedures to determine red blood cell (RBC) parameters, such as blood volume, cannot be employed in a population of several million pregnant women, children and newborns because of the danger of incorporating radioactivity in the growing organisms. Accurate blood volume determinations are important for the general screening, diagnosis and treatment of problem pregnancies associated with fetal growth retardation, premature labor and maternal hypertension. A potential important subset of this population includes at-risk newborns, for whom the RBC parameter measurements are additionally restricted due to the very small blood sample volumes available for diagnostic purposes. Recent developers in high sensitivity laser ionization coupled to a simple mass analysis device may provide the technical basis to determine these parameters using safe stable isotope tags. The specific aims of this proposed project are: 1) Develop methods to thermally atomize and ionize picogram and smaller quantities of chromium; 2) Evaluate the methods to determine chromium isotope enrichments in serum reference materials; 3) Conduct a pilot study to determine blood volume using chromium stable isotope tags; 4) Integrate the results of specific aims 1-3 to comprise the basis for a more comprehensive patient study and prototype commercial service instrument in Phase II.